O'Brien's catchy measured word patterns and basic counting lessons enliven this debut set on a carefree farm. After putting her chickens, horses and cows calmly to sleep, a ``farmer girl'' discovers that her four mischievous sheep have no intentions of snoozing. At that point matters go comically awry--the girl and her sheepdog follow the woolly insomniacs all over the pasture as they graze and get into trouble. The narrative's appeal lies in its cadences--counts of four on each page that invite readers to spot the restless ``black sheep, gray sheep, white sheep, four'' in each picture. McCarthy ( Buffalo Girls ; Ten Little Hippos ) provides conventionally cute, pastel-hued illustrations of the smiling, fluffy sheep and their fatigued but invariably patient keeper. This whimsical though slight look at a good-natured shepherd's many responsibilities possesses an agreeable guilelessness. Ages 2-6. (Oct.)